1. Field of the Invention
The present invention relates to the apparatus for recovering energy during the pressure let-down of high pressure reactor effluent, particularly to a reciprocating engine used for that purpose.
Prior Art
Many chemical reactions are conducted under conditions of high pressure. At some point in the process, this pressure is relieved or dissipated so that the product, unreacted components, etc., can be recovered. Not infrequently, considerable energy has been put into pressurizing the system and reactants. The conventional manner of operating such systems has been merely to lose the energy represented by the effluent pressure by reducing the pressure across a valve.
Most of the prior effort to recover this "process energy" has been concentrated on the design of a turbine through which the reaction effluents would be passed, as shown, for example, in U.S. Pat. Nos. 2,850,361 and 3,649,208. Such an approach may work in a single phase reaction system, however, in a multiphase system, particularly those wherein the pressure reduction is employed to cause phase separation, a turbine is generally unsatisfactory. Many difficulties exist in the design of such a turbine, because as the pressure is reduced, a liquid or solid phase separates from the gas and tends to coat turbine blades and plug passages. Turbine construction is such that imbalancing of the blades by random deposition of material thereon can cause failure of the engine.
U.S. patent application Ser. No. 070,566 filed Aug. 29, 1979 and now U.S. Pat. No. 4,288,406 issued Sept. 8, 1981, commonly owned herewith, which is incorporated herein, discloses a novel system and process employing a novel reciprocating engine for the recovery of energy from pressured reaction effluent. A reciprocating engine is generally a less delicate device than a turbine and is not incapacitated by some degree of fouling. The reciprocating engine which is used to recover the energy in the form of pressure from the reaction system is comprised of two or more cylinders, each having a piston or plunger slidably mounted therein and connected to a crankshaft either directly or indirectly.
Briefly, the reciprocating engine comprises at least one cylinder, said cylinder having an inlet and outlet port, said outlet being distal to said inlet port, means for opening and closing said inlet port, a piston movably mounted in the cylinder, having a conduit therethrough, means for opening and closing said conduit and a drive rod operably associating the said piston(s) to a crankshaft. Each piston is fitted with a valve which is biased open, thereby providing egress therethrough to the outlet in the cylinder. Opposed to each of the valves in each piston seated in the inlet is an inlet valve, which is biased toward the piston and which closes the cylinder. The cylinder is connected to the reactor through the inlet valve. As the piston makes its upward stroke toward the inlet valve in the cylinder, a portion of the piston valve contacts a portion of the inlet valve. The piston valve is forced closed and the inlet valve is then forced open. Effluent fluids then enter the cylinder in an expansion chamber forcing the piston downward, i.e., away from the inlet valve, and disengaging the contact of the two valves which allows the inlet valve to close. The piston valve opens when the pressure in the expansion chamber between the piston and the inlet valve is equal to the pressure adjacent to the outlets, thereby allowing the fluid to exit the expansion chamber as the piston repeats the cycle.
Each of the pistons is sequenced to provide the conventional reciprocating action.
In particular, the present system and the process and apparatus, are suited for the separation of multiphase effluent systems, wherein the pressure reduction is a means for separation of the phases, for example, the high pressure reaction of ethylene to produce low density polyethylene wherein a substantial portion of the ethylene is unreacted and is separated by depressuring the system whereby the polymer separates as a liquid phase and the unreacted ethylene gas is recycled to the compressors. In a typical reactor, the pressure may be reduced from about 2800 kg/cm.sub.2 to about 300 kg/cm.sub.2.
It is a particular feature that the present invention provides a substantial increase in energy recovery of the reciprocating engine.